Angiogenesis, the formation (sprouting) of new blood vessels from pre-existing vasculature, is impaired in aging. This impairment is detrimental to the healing of wounds and revascularization of ischemic tissues. Inspite of its clinical importance, the cellular mechanisms that regulate angiogenic sprouting, and the microvascular endothelial cells (ECs) and mural cells that comprise newly formed vessels, are poorly understood in aging. The details of angiogenic sprouting are difficult to observe in vivo due to tissue thickness and the presence of stromal and inflammatory cells. Moreover, the tip region of each sprout, which is a critical area of investigation, is rarely found in tissue sections. We propose that vascular segments cultured ex vivo are the most effective means to study the details of angiogenic sprouts. In this model, vascular segments are isolated from tissues with an abundant microcirculation (e.g., adipose) and placed in 3-dimensional collagen gels that simulate interstitial matrix in vivo. The segments serve as "parent vessels" from which sprouts are generated and imaged. We propose to use microvessels from young and aged mice, in conjunction with novel culture methods and modern image analysis, to refine and extend the capabilities of the microvascular sprout model. Our hypothesis is that aged sprouts will show an increase in the number of mural cells, relative to ECs, and that the formation of cell-cell contacts is delayed. We also postulate that deposition of basement membrane will proceed normally, but there will be deficient matrix turnover at sprout tips. Aim 1 will define the cellular components, their morphology, orientation, and the influence of migration, relative to proliferation. Aim 2 will determine the influence of age on formation of cell-cell contacts, the deposition of basement membrane proteins, and the expression/activity of matrix degrading metalloproteinases. The results will define the key points of deficient microvascular growth in the aged and also develop the microvessel sprout model into a highly useful tool for studies of vascular growth in general.